1. Field of the Invention
The present invention relates to thermal transfer image-receiving sheets which are used in superposition on thermal transfer sheets.
2. Description of the Related Art
As an image-forming method utilizing a thermal transferring process, a method called “sublimation-type thermal transfer system” is known in which a thermal transfer sheet comprising a substrate sheet such as paper and a plastic film supporting thereon a sublimable dye as a recording material and a thermal transfer image-receiving sheet comprising paper or a plastic film having thereon a layer for receiving the dye are superposed on each other so that a full-color image is formed. Because of use of a sublimable dye as a color material, this method can adjust the density and the tone freely by dot so that it is possible to clearly form a full-color image faithful to an original document on an image-receiving sheet. This method is therefore applied to color image formation in digital cameras, videos, computers and the like. The resulting images are of a high quality comparable to silver salt photographs.
The images to be formed by printing by means of the sublimation-type thermal transfer system are required to have a high image quality and also a light resistance comparable to that of silver salt photographs. A major approach for achieving such light resistance is to print yellow, magenta and cyan on a thermal transfer image-receiving sheet, followed by lamination of a protective layer thereon.
On the other hand, with respect to a binder for use in thermal transfer sheets, acetal-based resin has been mainly used. In such a case, use of a polyester-based resin in a receiving layer causes a problem in that it is difficult to secure a proper releasability between a thermal transfer sheet and an image-receiving sheet because the acetal-based resin and the polyester-based resin are liable to be thermally fused during printing.
In order to secure a sufficient releasability, techniques in which a large amount of releasing agent such as silicone is added to the receiving layer are known (for example, Japanese Patent Application Laid-Open No. 08-108636, Japanese Patent Application Laid-Open No. 2002-264543). However, these techniques cause problems such as a poor printed-article preservability and a failure in transferring the protective layer in spite of a sufficient releasability, resulting in significant difficulty in maintaining a proper balance between the releasability and the protective layer transferability when a polyester resin is used in the receiving layer.
Printers of sublimation-type thermal transfer system may be placed outdoors as an stations for outputting digital photographs, identification photographs. Therefore, image-receiving sheets are required to be superior in the releasability and the protective layer transferability even after their preservation in severe environmental conditions, such as high temperature, high temperature and high humidity, or low temperature.